The air nomads were the only bending group to be all benders because of their culture being so spiritually stimulating. So since the culture is pretty much dead and the only people left to mate with are non airbenders and not part of the culture then the likelihood of birthing an airbender was reduced to more closely resemble the other elements.

They keep it alive on his island, they all wear airbender robes, are vegetarians, and their are the air acolytes who train in the spiritual side of air bending even though none of them are air benders.

The spirituality aspect of the airbenders culture is what makes them benders, not a dominant trait. The idea is that if they have bender blood, they will be a bender, however aangs children could be air or water benders. It's possible that a child born of a union of an airbender and non-bender would always result in an airbender, as it has with tenzin's first 3 children